The invention pertains to a process for thermal activation of zeolites by percolating a hot gas through a bed of granulates of these zeolites and, preferably, to a continuous process for such activation. It applies to zeolites which are degraded under the combined action of water vapor and temperature, and more particularly to the forms of type A and X zeolites exchanged by potassium or calcium. It produces activated zeolites with high adsorption capacity.
Zeolites are crystallized aluminosilicates possessing a calibrated porous network. Because of the exceptional porosity of their structure, they constitute a veritable screening tool on the molecular scale by the selective adsorption of molecules, the molecular selection being based either on steric effect or on polarity effect. Their name of "molecular sieves" is a statement of this property.
The industrial applications of zeolites are widely varied and include the processes of separation, purification and catalysis. Among the most valuable zeolites are zeolite A, the sodium hydrated form of which (zeolite NaA) has the formula: EQU Na.sub.2 O Al.sub.2 O.sub.3 2SiO.sub.2 (4 to 5) H.sub.2 O
and zeolite X, the sodium hydrated form of which has the formula:
Na.sub.2 O Al.sub.2 O.sub.3 (2 to 3) 2SiO.sub.2 (5 to 6) H.sub.2 O.
Zeolites have been broadly described in the literature and more specifically in the articles by Breck. They are easily exchanged in aqueous media by numerous cations, which modifies their adsorption properties. This is also the case when replacing the sodium with potassium, the pore size of the zeolite NaA is reduced by 4A.degree. to 3A.degree.. Thus, one obtains zeolites designed 3A or KA. On the other hand, by calcium exchange of the zeolite NaA, the pore size is increased by 4A.degree. to 5A.degree. and one thus obtains zeolites designated 5A or CaA.
Some uses of such exchanged zeolites A or X are presented below:
(i) Utilization of the zeolite CaA to separate normal paraffins from charges containing branched, aromatic and olefinic hydrocarbons. In such processes, the zeolite is brought into contact with the mixture of hydrocarbons, most often in vapor phase, where it is preferentially charged in linear molecules. After this, one recovers the adsorbate either by increasing the temperature or by reducing the pressure, or by scavenging with a compound acting as a desorbant.
(ii) Utilization of the zeolite CaA to separate permanent mixtures of gases (for example, O.sub.2 /N.sub.2 mixtures in the enrichment of air with oxygen, H.sub.2 /N.sub.2 mixtures in the purification of hydrogen). The processes in which these zeolites are used are known by the name of PSA (from the English name of the process "pressure swing adsorption"). They function according to the adsorption/desorption cycles by varying the pressure under isothermal conditions. (iii) Utilization of the zeolite NaX in decarbonation processes for gas desulfurization treatments.
(iv) Utilization of the calcium form CaX for the separations of N.sub.2 /O.sub.2 mixtures.
After having carried out the ionic exchange and having agglomerated the resulting product (in general with about 20% by weight of a clay binder), it is necessary to proceed with elimination of the water incorporated in the structure to give the zeolite its adsorption properties. The best adsorption capacities that can be obtained are those of zeolites activated in the laboratory by degassing the dehydrated zeolites under vacuum. Industrially, one proceeds with thermal activation in ovens. It should be noted that the products thereby obtained have remarkably low adsorption capacities. This activity deficit, which is attributed to a hydrothermal degradation phenomenon, is very perceptible, namely, as concerns the adsorption capacities of sieves 5A and X for nitrogen, sieves 5A for carbon oxide, sieves 5A for paraffins and sieves X for carbon dioxide gas.
Among the processes which have been described to remedy this loss in activity, some tend to reduce the sensitivity of the exchanged zeolite to hydrothermal degradation by acting on the exchange conditions. They are, for example
(i) Patent No. DD 226,864 (VEB) which claims a calcium exchange process for the zeolite 4A carried out at 25.degree. C. on a zeolite NaA having a pNa (sodium proportion based on the cologarithm of its molar concentration) ranging between 0.8 and 1.9 and a HN.sub.3 adsorption capacity ranging between 150 and 400 mL/G; and
(ii) U.S. Pat. No. 2,908,549 (TEXACO) which discusses a calcium exchange process in the presence of solid lime and a calcium salt at a pH value maintained around 11 during the exchange, the calcium salt being indifferently the chloride or the formate.
Others act on the activation conditions, such as East German Patent No. DD 239,533 (Karl Marz University, Leipzig), which describes a process for activation of the zeolite 5A in the course of which a strong gaseous scavenging is carried out in a bed activated by a turbulent movement, while respecting the slow rate of temperature rise in the maximum water elimination areas.
However, nevertheless, these processes only provide zeolites whose adsorption capacities remain remarkably lower than the anticipated theoretical capacities.